A recently quenched galaxy 700 million years after the Big Bang

Local and low-redshift (z < 3) galaxies are known to broadly follow a bimodal distribution: actively star-forming galaxies with relatively stable star-formation rates and passive systems. These two populations are connected by galaxies in relatively slow transition. By contrast, theory predicts that star formation was stochastic at early cosmic times and in low-mass systems. These galaxies transitioned rapidly between starburst episodes and phases of suppressed star formation, potentially even causing temporary quiescence—so-called mini-quenching events. However, the regime of star-formation burstiness is observationally highly unconstrained. Directly observing mini-quenched galaxies in the primordial Universe is therefore of utmost importance to constrain models of galaxy formation and transformation. Early quenched galaxies have been identified out to redshift z < 5 and these are all found to be massive (M* > 10^10 M⊙) and relatively old. Here we report a (mini-)quenched galaxy at z = 7.3, when the Universe was only 700 Myr old. The JWST/NIRSpec spectrum is very blue (U − V = 0.16 ± 0.03 mag) but exhibits a Balmer break and no nebular emission lines. The galaxy experienced a short starburst followed by rapid quenching; its stellar mass (4–6 × 10^9 M⊙) falls in a range that is sensitive to various feedback mechanisms, which can result in perhaps only temporary quenching.

Tobias J. Looser, Francesco D'Eugenio, Roberto Maiolino, Joris Witstok, Lester Sandles, Emma Curtis-Lake, Jacopo Chevallard, Sandro Tacchella, Benjamin D. Johnson, William M. Baker, Katherine A. Suess, Stefano Carniani, Pierre Ferruit, Santiago Arribas, Nina Bonaventura, Andrew J. Bunker, Alex J. Cameron, Stephane Charlot, Mirko Curti, Anna de Graaff, Michael V. Maseda, Tim Rawle, Hans-Walter Rix, Bruno Rodríguez Del Pino, Renske Smit, Hannah Übler, Chris Willott, Stacey Alberts, Eiichi Egami, Daniel J. Eisenstein, Ryan Endsley, Ryan Hausen, Marcia Rieke, Brant Robertson, Irene Shivaei, Christina C. Williams, Kristan Boyett, Zuyi Chen, Zhiyuan Ji, Gareth C. Jones, Nimisha Kumari, Erica Nelson, Michele Perna, Aayush Saxena, Jan Scholtz